Marangoni dryers have been used in the past to dry semiconductor wafers that are being processed, such as by liquid baths.
Marangoni drying is based on surface tension gradient forces and is an ultra-clean drying process. In this technique, a volatile organic compound, such as isopropyl alcohol (IPA), with lower surface tension than water, is introduced in the vicinity of a substrate semiconductor wafer in the form of a vapor as the substrate wafer is slowly withdrawn from the water. This assumes a water bath. As the small quantity of alcohol vapor comes into contact with the continuously refreshed water meniscus, it is absorbed into the water and creates a surface tension gradient. The gradient causes the meniscus to partially contract and assume an apparent finite flow angle. This causes a thin water film to flow off the substrate and leaves it dry. This flow also will assist in removing non-volatile contaminants and entrained particles.
In a known Marangoni dryer, semiconductor wafers are submerged in deionized water (DI water). An airtight cover or lid is placed over the bath that contains the submerged wafers, creating a sealed process chamber. Air is purged from the chamber with nitrogen gas (N2) and an IPA (isopropyl alcohol) bubbler produces IPA vapor that is introduced with N2 into the chamber. A thin layer of IPA forms on the DI water surface. Wafers are slowly passed through this IPA layer and onto a wafer carrier positioned in the IPA/N2 vapor environment.
In another known approach, the wafers are submerged in a bath of a dilute concentration of hydrofluoric acid. Following treatment by the acid bath, deionized water is introduced into the bath to flush out the hydrofluoric acid through an overflow. Marangoni drying, as described above, is then performed.
In the absence of Marangoni drying, the surface tension of DI water on semiconductor wafer features of small size (e.g. 30 nanometer features) can cause feature damage by clinging, pulling, bending or breaking structures due to this surface tension.
Although Marangoni drying of semiconductor wafers offers advantages, semiconductor wafers processed with Marangoni dryers known to the inventor have resulted in the depositing of materials on processed dried wafers, such as liquid that leaves behind water marks on the wafers. These water marks may have gone unnoticed at 120 nanometer feature sizes and above. However, they are visible using, for example Surfscan lasers, when looking at wafers having 75 nanometer or smaller features. These deposits can result in problems for wafers with small features.
Therefore, a more effective dryer and method of drying semiconductor wafers using Marangoni drying techniques is needed.